I propose to investigate the regulation of light-dependent membrane channels in rod photoreceptor outer segments, using a new electrophysiological technique: single-channel, or "patch" recording. This technique will allow the direct observation of current through single membrane channels, and the direct observation of current through single membrane channels, and the direct control CA++ concentraion at the inside membrane surface. It will also permit the introduction of cyclic GMP, as well as putative enzymatic components of the visual transduction mechanism, into the cytoplasm. A statistical analysis of the effect such manipulations have on the open and closed times of the channel will provide a more detailed model for the regulation of channel activity by putative second messenger substances. These experiments are designed to identify the second messenger substance that acts as an intermediary in visual transduction, and to elucidate the nature of its interaction with the membrane channels. They will significantly deepen our understanding of sensory transduction in the visual system, which may serve as a general model for chemical regulation of the electrical behavior of nerve cells.